Using SWA cable

[andrewpreece]

I bought a metal enclosure box, one with knockout holes in it IIRC, and used it as a cable changing box. If you buy the SWA cable glands you will be able to bend the steel armouring back over the tapered collet thingy ( need to trim it just right though ), and screw the gland together tight, compressing the steel armouring and thus getting what I believe to be a very good bonding. I think the cable gland kits also come with a terminal washer, which I assembled onto the gland as I screwed it into the cable-changing box with the nut that is supplied, and soldered a wire onto the terminal washer , the other end of the wire going to a stud on the box, sort of belt and braces really. I used a metal cable-changing box at both ends actually, as I didn't consider the pvc boxes to be strong enough to take the rather stiff SWA to, and with metal you get a direct earth bond as well. Use some heavy duty terminal block inside the CCB to connect the cables together, at least, that's how I did it. You will need a grommet for the PVC cable as it exits the ccb.

Andy.

[jim_in_sussex]

Planning to bring my garage supply (which has a workshop section) up to date in Christmas week. (last legal throw before part P)

A thread ran a few weeks back on running a supply cable from a house to a (detached) garage & SWA cable was suggested. To take up that suggestion advice is needed as it is a long time since I used SWA cable (& then only once).

1. Rating: It would be buried (as per above thread) about 450mm (18ins) underground (hard drive/path) - about 4m as the crow flies, but 7m-12m after deviations round, up & down. Is it correct to use IEE table 4D1A/ref method 1? (the illustrations for method 1 don't refer specifically to underground use).

1. Ducting: Instead of burying it in bare soil it looks more sensible to run it inside a duct, but attempts to source underground ducting have failed. Does anyone know of a source, or failing that, could semi-flexible 40mm waste pipe be used? If SWA is ducted, how does it affect current rating?

2. Termination: The house end is no problem as that connects into a metal enclosure which I know how to connect, but how do you connect the garage end into a standard plastic consumer unit? Is there a special gland for this? Or do you connect into a service block of some type & then run the last 12 ins or so in tails?

1. Earthing: I'm not completely happy about relying on a non-copper earth (ie the steel armour). Instead can I use 3 core SWA & use the

3rd core as earth to supplement the armour? Common advice seems to be to avoid using the house earth, but if possible I'd like to avoid the work involved in installing an earth rod. The house supply is 100amp PME/TN-C-S.

1. Cable size: nominally 6mmsq cable is adequate for the garage load (ring + freezer radial + lights), but am wondering about using a larger cable - eg 16mmsq. There'd be 2 benefits - much lower volt drop, plus less light flicker when a machine starts. Relative to the whole project the extra cost is small, but are there any practical snags? Does anyone know the outside diameter of these cables?

2. Finally looking up the ratings for these cables in the IEE regs & comparing with the TLC catalog, why the differences? eg TLC show swa

16mmsq 2 core (6942x) as 91amps, IEE table 4D4A shows 89amps. In fact most cable amp values given by TLC are lower than the IEE tables give.

Many thanks for all help offered.

[Lurch]

On 17 Dec 2004 14:59:49 -0800, jim_in [e-mail] (jim_in_sussex) strung together this:

I don't have any regs to hand, I'm sure someone else will.

Why? Unless you're likely to be pulling it out then bury it direct. There's no mechanical advantage to be gained from running SWA in a duct. The current rating will be the same.

Two ways, first is to terminate directly into the PVC enclosure and the second is to terminate into an external PVC or metallic enclosure then run from this to the CU with T&E or something similar.

I always use 3 core SWA and use the 3rd core as an earth. Common practice really. Earthing, I'm not going to get into this one.

6mm about 20mm OD, 16mm about 30mm OD. You can use either, I'd go for anything from 6mm upwards for a 32A supply over the distance you've got.

Email TLC and ask, could be a number of reasons.

[Dorothy Bradbury]

XLPE probably (modern higher temp rating). Think it was XLE that deteriorated over time for HV links & went boom.

No need, but do lay a "cable buried below". o Ducting is good at filling with water o Ducting =/= run net/phone cable next to power cable o Ducting = run net/phone cable in flex-conduit next to power cable

Consider how long your ring-main loop can be serving a typical 50m^2 area (subject to voltage drop & earth disconnect time). Your proposed run is only a few metres, and XPLE cable rating is higher than PVC FTE, so 16mm^2 may be somewhat overkill unless you can on a shower :-)

Note the XLPE rating (which is re higher temp insulation IIRC) may be a factor in your sizing of FTE to feed it from the CU or from it to other CU.

If you are using big enough machinery to really cause the lights to dim: o Consider a non-maintained emergency light in the garage (£19-22)

---- whole-garage RCD = lights go out & excitement begins o Consider a garage CU with RCBO for ring, plain MCB for lights/freezer

---- new MK/Hager/MG 1-module 32A RCBO is £19-26 are regular on Ebay

[andrewpreece]

Only 12 hours out, picky, picky, picky... :0)

Andy.

[John Rumm]

I take it you found TLCs voltage drop calculator?

handy for SWA I find.

[Lurch]

On Fri, 17 Dec 2004 14:46:23 -0000, "andrewpreece" strung together this:

Your clocks wrong, thrash your self with 2.5mm T&E now.

[Lurch]

On Sat, 18 Dec 2004 00:34:35 GMT, "Dorothy Bradbury" strung together this:

Don't reply to me, I didn't ask the question. Bloody OE users....

No, you use seperate ducts for power and low voltage services.

[Mike Harrison]

SWA is designed to be buried - save the ducting for phone, network, doorbell, whatever. Remember to run some string through for future pulls.

You can get metal glands with tags for terminating to the armour.

That's what I did - use the 3rd as earth, but also earth the SWA, in case it ever gets a spade/digger through it..!

I think 16 would be a nightmare to handle - the bend radius is probably of the order of a foot or two. Maybe split the difference and use 10.

[Lurch]

On Sat, 18 Dec 2004 11:03:10 GMT, Mike Harrison strung together this:

Sorry Mike but is there any chance you could adjust your line length\character width to 70 characters or thereabouts? It makes posts rather difficult to read. Thankyou.

[Owain]

"jim_in_sussex" wrote | 3. Termination: The house end is no problem as that connects | into a metal enclosure which I know how to connect, but how | do you connect the garage end into a standard plastic consumer | unit? ... Common advice seems to be to avoid using the house | earth, but if possible I'd like to avoid the work involved | in installing an earth rod. The house supply is 100amp PME/TN-C-S.

AIUI PME earths should not be exported from the house. It would be normal to use 2-core SWA, use an earthing gland at the house end so that the armour is earthed, use an insulating gland at the garage end so that the armour is insulated. The CU is then earthed to an earth rod (TT) and all circuits should be RCD protected. If you use power tools in your workshop you should consider discrimination between lighting and power circuits, so that in the event of an RCD trip you are not left in the darkness with power tools spinning down. An alternative might be a couple of self-contained emergency lighting units.

Owain

[Dorothy Bradbury]

o Yes I similarly loathe Outlook (in this case) o One reason I'm migrating to Linux - that and multiple O/S fees

Even if the LV is conduited? I guess I'm thinking of the MK etc skirting trunking. That allows LV & Mains in the same trunking due to separated design, ie, it's little more than several trunking c/sections extruded as one piece.

Thanks for the correction.

[Lurch]

On Sat, 18 Dec 2004 17:49:59 GMT, "Dorothy Bradbury" strung together this:

Apology accepted! ;-)

Well done. I started doing that a while ago.

Theoretically, you're correct, but if there is a duct with an SWA in it then it is a mains duct, them's the rules! If you were to place a duct within a duct then you could do it but you don't need to, you would just bury the SWA direct and use the duct for LV.

[BigWallop]

But that means the post you sent today didn't arrive until yesterday. So you've already missed all the good parts. :-)

[Andy Wade]

No, for XLPE insulated SWA (to BS 5467) use Table 4E4A. Current rating is rarely an issue for SWA uses such as this; voltage drop is usually the limiting factor.

SWA is _intended_ for direct burial, as others have said. If you want to use duct you can get 6m lengths of push-fit service ducting from your local builder's merchant.

For 3-core the o/d for 6mm^2 is 17.0mm and for 16mm^2 it's 22.4mm (and

19.9mm for 10mm^2). [figures from Pirelli data sheet]

The TLC figures might be out-of-date info for the older BS 6346 PVC insulated SWA cables. Table 4E4A gives 110 A (method 1) for 16mm^2. I advise ignoring any cable current ratings given in re-sellers' literature as they're frequently wrong. The Pirelli data sheet gives ratings of 141 A for direct-in-ground, and 115 A in duct, the latter agrees with 4E4A method 12.

[jim_in_sussex]

Am unable to post against the above original posting for some reason - google refuses to retrieve the original message, so:

first many thanks to all posters for the advice: much appreciated & has put me on course to get up to date on SWA.

Andy Wade commented:

-------

For 3-core the o/d for 6mm^2 is 17.0mm and for 16mm^2 it's 22.4mm (and

19.9mm for 10mm^2). [figures from Pirelli data sheet]

----- Andy, please can you supply a pointer to this data sheet - I looked in pirelli.co.uk but couldn't find a relevant table

looks to me that 16mmsq is getting a tad too large to handle: so am thinking about10mmsq which will give a bit of rating leeway & it needs to go into bare soil.

If I export the house earth using the 3rd 10mmsq conductor as earth give an adequate earth? The alternative is to make the garage a TT supply (with an earth rod) which implies all RCD & I'm not enthisastic about that.

thanks again

[BigWallop]

It's best to have some RCD protection in the garage, with at least one light that runs from a normal MCB breaker out with the RCD. The garage will be the most likely place to use large power tools and things, so an RCD is safer for this type of installation. You also shouldn't really transport the earth from the house, as any trip off in the garage through an earth conductor may also cause a trip off in the house supply as well. The earth on the sub mains cable should be made through the armour, and an earth conductor if you wish, but it is only to protect the cable running to the garage if anyone hits it with a spade or pick axe.

Lurch has drawn my attention to a Clipsal Garage Consumer Unit which is an ideal system for this type of power supply to an out building. A picture of it here:

from the toolstation website shows what you get.

[John Rumm]

What would be the mechanism for that? (Assuming the house is not wired with an old ELCB)

[BigWallop]

"May" I think is the operative word I used. I don't know how, what or where the house is wired or supplied, That I leave to OP to find out about through their own research into electrical installations. I can't be there when they do the job, so I always advise on the safe side. A sub mains cable, which the OP is installing, only needs to be Live and Neutral. The cable itself must also be mechanically protected against damage from out with the cable. It also needs an automatic safety isolation device from its supply source to protect is against short circuit.

I would always advise that a sub mains supply to an out building be made through its own separate isolator taken from the main supply head end to the property. This makes the sub mains supply an entity of its own which doesn't (shouldn't) interfere with any other parts of the total installation. I always advise that the earthing be made through a separate system to the main property, as this also leaves the sub mains supply as a totally separate entity to the main electrical supply system.

If it's a totally separate supply system, then it can be treated as is its own separate installation away from any other parts of the main building. I've always made my own installations of this sort, in this way, and I've never had a failure or created problems to any other parts of the main system. I think this is the safest and best way to do these types of sub mains electrical supply installations.

They are easier to install if thought of as a totally separate system from the main house. They cause less problems when made a totally separate system from the main house. I think a sub mains electrical supply should be a totally separate system from the main house supply. Maybe that's why they're called sub mains electrical supplies.

[Andy Wade]

[Pirelli data]

Try

and yes, their web site is truly abysmal.

10mm^2 sounds about right to me.

Yes, so far as conductor size is concerned (and the armour alone of a

2-core cable may well be adequate, but that would require calculation). Remember that if you export the house earth (especially as you're on PME) you must create an equipotential zone in the garage, which means main-bonding any incoming service pipes or accessible metalwork that's in contact with the ground. If the floor is dry and your work is mostly internal then this is an acceptable solution, IMO. But if the garage has a damp solid floor and/or you are likely to use portable equipment of Class 1 (earthed, 3-core flex) construction outdoors from this supply then the TT method is preferable.

See above. Sockets in the garage will almost certainly fall under the "likely to be used for portable equipment outdoors" heading, so must be

30mA RCD protected in any case.